1. Field of the Invention
This invention is in the field of zinc metallurgy using a leaching process and a pyrometallurgical volatilization process to recover zinc values.
2. Description of the Prior Art
The industrial recovery of zinc by means of leaching and electrolysis has attained more and more importance since its development at the beginning of this century, particularly since high quality zinc ores are particularly amenable to the process and particularly with reference to their impurities or contaminants. Such impurities include iron and silicic acid as well as arsenic, antimony, cobalt, nickel, germanium, chlorine and fluorine. Since these elements are almost always included in the zinc minerals such as zinc blende or sphalerite, in the past fifty years the commercial utilization of the zinc electrolysis process had to deal with the problem of making these impurities innocuous.
The relative economy of the recovery of zinc by means of electrolysis depends to a large extent on whether it is possible to remove the impurities or contaminants by a combination of roasting and leaching.
When the zinc electrolysis method was first being utilized commercially, the refiner could still fall back upon particularly high quality in pure zinc ores and ore concentrates, and did not have to concern himself with the effect on the environment caused by depositing residues. Accordingly, only a simple neutral leaching was required in such instances. However, when zinc ores were reduced in quality and contained substantial amounts of iron, for example, and the refiner had to deal with fluctuations in the composition of the ore, the feasibility of utilizing the electrolysis method was problematical, if not actually partially uneconomical.
The problems involved begin with the primary leaching of the roasting material used. When the oxide zinc carrier material is treated with diluted sulfuric acid at a pH of about 3 to 4, the iron which had previously gone into solution after oxidation precipitates as ferric hydroxide. When the ferric hydroxide precipitates as a flocculent precipitates, it carries with it an appreciable part of the other contaminants such as arsenic, antimony and germanium. In order to achieve this important effect, however, additives must be added which decrease the probability that the zinc oxide contained therein is completely dissolved. Accordingly, outputs of only about 70 to 90% at the most were obtained, and the leaching residues remained with uneconomically high undissolved portions of zinc.
In order to cut down the cost of this type of process, it was then suggested that the recovery of zinc values could be improved by using several leaching steps connected in series with modified leaching and precipitation conditions, together with much higher acid concentrations in some stages, and increased temperatures as well as oxidation of the material in gaseous form. This method was particularly feasible after it was found that codissolved quantities of iron in the form of jarosite or goethite could be precipitated out of solution as an easily filterable residue under predetermined conditions.
With these types of methods, however, it was found that the multiple step leaching process required a very substantial amount of investment and operational costs. For example, the leaching liquid must be thickened in a separate vaporization step to the required very much higher concentration for electrolysis. Furthermore, the plurality of leaching steps requires a very much more complex installation of apparatus in one leaching plant, leading to a very much more cumbersome installation in terms of supervision and control.
In addition, a particularly serious disadvantage resulted from the fact that the waste products produced in the case of the jarosite and/or goethite method was a material which provided appreciable problems in disposal because of the injurious nature of the substances contained therein such as arsenic, antimony, thallium, bismuth and the like. These materials are easily washed out of the residue by rain water and therefore when deposited in the open, contaminate the ground water. For these reasons, installations of the multiple step leaching of zinc ores today require considerable expenditures for the proper disposition of such residues, so that the economic advantages which previously existed are no longer present.
A different course was pursued with the pyrometallurgical treatment of the leaching residues. A series of treatment procedures were suggested, tested and put into large scale operation. The latter included rolling- and direct-rolling methods and extended through the most varied shaft, semi-shaft and sintering furnaces, with or without pre-compression of the feed. It was always the aim to improve the production of zinc with the hydrometallurgical leaching method by combination with the pyrometallurgical volatilization method whereby leaching and volatilization were employed in cyclic processes.
In accordance with this improved method, the leaching residue after drying with the addition of carbon, was mixed with slag forming additives at temperatures above 1000.degree. C., and melted in a reducing atmosphere. The zinc content was volatilized as metal vapor, and subsequently oxidized in the gas stream and recovered again in dust collection aggregates. The non-volatile metallic and slag forming constituents resulted in a fusible or molten phase made of stone and slag. The high zinc containing oxide-sulfate dust recovered was again supplied to the leaching step.
Commencing in the 1950's, this method of treatment was regarded as disadvantageous for the reason that the dried leaching residue was present in pulverulent, very finely divided grains, requiring pelletizing, briquetting or sintering which added significantly to the cost. In addition, the thermal processes required expensive energy sources such as metallurgical coke or oil. Since the process involved a multistep thermal metal recovery it necessitated an increased size of production in the zinc plants which rendered the entire procedure uneconomical. These procedures were thus harmful to the environment, expensive in operation and maintenance and required high investment costs.